Classifier for material reduction mills



Dec. 14, 1954 3 Sheets-Sheet 1 Filed Aug. 14, 1951 //v vmv rue DA via [Me's ra/v Dec. 14, 1954 0. WESTON 2,696,903

CLASSIFIER FOR MATERIAL REDUCTION MILLS Filed Aug. 14, 1951 5 Sheets-Sheet 2 Dec. 14, 1954 D. WESTON 2,695,908

CLASSIFIER FOR MATERIAL REDUCTION MILLS Filed Aug. 14, 1951 s Shets-Sheet s A rra ,e/vzvs I D [(10 h zsra/v Byw- United States Patent CLASSIFIER FOR MATERIAL REDUCTION MILLS David Weston, Toronto, Ontario, Canada Application August 14, 1951, Serial No. 241,807

Claims priority, application Canada August 1, 1951 14 Claims. (Cl. 209-26) The present invention relates to an apparatus for recovering from an air stream entrained solid particles having a predetermined size-Weight factor, and more particularly it relates to a classifier for use in connection with material reduction mills from which product is withdrawn in a stream of air whereby the separation and recovery of particles of particular size-weight factors from the air stream may be accomplished.

In the art of comminuting solid materials such as rock, ore and the like, it has long been recognized that basically it is desirable to produce a subdivided product in which the valuable constituents are present as essentially primary crystals, since the reduction in size of the individual particles to a further degree normally does not result in any benefit insofar as subsequent recovery of the values is concerned, but may on the other hand result in the formation of slimes, rendering subsequent recovery more difiicult. On the other hand, if the material is not reduced to the point where the values are in the form of primary crystals, the particles themselves will be combined aggregates of gangue materials and values which cannot be separated from each other by normal mechanical means such as gravity concentration. In many reduction operations advantage may be taken of the fact that the basic size of the crystals of gangue materials differs from the basic crystal size of the values, and accordingly it has been found possible in many cases to reduce all of the material substantially to its primary crystalline state and then by making a separation of the product on the basis of particle size, to separate the product into a concentrate rich in values and a tailing containing most of the gangue materials.

There are in addition a number of instances where values occur in relatively large bundles of primary crystals and where it is desirable insofar as possible to separate these bundles from the gangue without breaking them up. One such case, for instance, is asbestos, which normally occurs as densely packed bundles of substantially pure asbestos fibres in a host rock having a relatively fine primary crystal structure. In recovering asbestos fibre, the relative value of the pure asbestos recovered primarily varies with the length of the fibres, the relative value of the fibres of /2 inch length and over being many times the relative value of fibres, say of under inch in length. As is well known, however, primary asbestos fibres are comparatively delicate and any overgrinding of them will reduce their length and consequenty their relative value per unit weight. Moreover, since primary crystals of asbestos have the form of exceedingly slender filaments and the air resistance of each individual filament is great, having regard to the weight of the filament, separation of primary fibre from granular fines in the usual air separators, cyclones and the like, is very difficult, the result being that in order not to lose a substantial quantity of product the asbestos is normally collected as a relatively impure mixture of the granular and fibrous material, which then has to undergo a complicated cleaning process during which the granular material is eliminated. There has, accordingly, long been a recongnized need in the art for a means of separating and recovering bundles of asbestos fibre as soon as they are set free during a reduction op eration and before the individual bundles have been broken down into primary fibres.

Similarly, in the comminution of various other ores where relatively large particles, of values occur, it may 2,6953% Patented Dec. 14, 1954 ICC be advantageous to recover these large particles as they are freed, and to form a coarse concentrate from them.

The present invention is adapted for use in connection with material reduction mills of the type wherein the product is removed from the mill in a stream of air and has as its principal object the provision of a classifier arranged to form a part of the air circuit in a mill of this type which is capable of removing from the air stream all particles having a size-weight factor above a predetermined value, and recovering therefrom such particles as are within a given size range.

It is also an object of this invention to provide such a classifier which is essentially simple in construction and requires a minimum of maintenance during operation.

It is a further object of the invention to provide such a claissifier which may be arranged in series with other such classifiers to provide a series of products within different predetermined size ranges.

Other objects and advantages of the invention will be apparent as the specification proceeds.

A classifier according to theinvention is generally composed of two components of air duct of generally curvilinear or rounded cross section arranged with their axes generally in the same plane and intersecting at an angle at the plane of intersection. The first of said ducts, which is connected to the air circuit at a plane of intersection at the upstream end of the classifier, is of expanding cross section so that the plane of intersection of the ducts has a cross sectional area which is at least about 1.5 times, and preferably between about 1.5 and 3 times, the area of the plane of connection of the first component at the upstream end of the classifier with the air circuit. The angle of intersection of the two components of duct is a critical factor in achieving accurate classification in the apparatus of the invention. In the first place, the upstream component of duct in the classifier must be arranged so that the bottom of it makes an angle of at least about 38 with the horizontal in order that material separated from the air stream will flow back down it under the influence of gravity. In the second place, entrained particles within the size range which it is desired to remove from the air stream in the said first component of duct when carried along past the plane of intersection of the two components should strike the outer wall of the second component at a sufiicient angle at least momentarily to stop movement thereof in the direction of the axis of the second of said duct components so that, with the air stream at that point having a velocity which will not be suificient to carry them on upwardly out of the classifier, they will be permitted to flow back down along the lower surface of said first component of duct from where they may be removed through a suitable opening covered by a foraminous member having a collector beneath it.

In order for the above mentioned separating action to take place, it has been found that the angle between the axes of thetwo components should not be more than about 128 and the axis of the first component when extended should intersect the outer wall of the second component to form an acute angle with the tangent plane at the point of intersection of not less than about 38. On the other hand, if the particles, once stopped by impingement upon the outer wall of the second component, are not once again to be picked up by the air stream and carried out of the classifier the velocity of the air stream at that point must be materially less than the velocity of the air stream at the plane of intersection of the classifier and the air circuit. Accordingly the cross sectional area of the second component of duct at that point must be greater than the cross sectional area of said plane of connection. 7 In order to eliminate the possibility of the air stream having sufficient velocity to re-entrain the particles which it is desired to separate in the classifier the angle betwen the axes of said two components should not be less than about 94. In practice it has been found that an angle of intersection of the axes of the ducts which is between about and is satisfactory for most purposes, although the optimum angle will depend to some extent upon the nature of the material being reduced and the nature and size of the particles which it is desired to recover from the air stream in the classifier.

9-3 Since the whole action of the classifier according to the invention is based upon the effect produced in the region of the intersection of thetwo components of duct, it is .important.that the .first.component.of duct should be sufficiently. long for..the..air. stream as. it. approaches the .planerbf connection-=10 .have reacheda substantialequi- .Illibri-um .offlow, and inl'thisconnection it has been found .Z'that the leng'thi'of the axis ofl'thelfirst component between 'ithec'plane ofconnection and the plane of intersection 1 should?:be .atlleastl2.4.ttirnes the. mean diameter of .the ....plane [of connection.

L' Similarly, .it.. is important that" the second component .of duct-be sufficiently long .to embrace the whole zone of separation of particles from the air stream, and in :.this connection. it has beenlfound' desirable to havethe second component at leastllS times as long .as-.the mean "diameter; of the plane ofv intersection.

..The invention'will be more. fully understoodby' referencertolthefollowing' detailed specification taken in conljunction with the accompanyingdrawings which 1 illus- "trate a number of embodiments ithereof.

--In thedrawings:

Figure 1 is a general'view of. a material-reduction mill equipped with a 'class'ifier "according to"the'invention illustrating the general" arrangement of the'fclass'ifier. in relation 'tolthe' other components 'ofthe' mill;

iFigureZ is a"detailedt's'ide"view of a classifier accordf ing' Ito the "invention; partlybr'oken awayf'to illustrate details "thereof;

Figure 3"is a fragmentary view of" the" classifier shown 'iri'Figure 2; with the products collectorirernoved to.'illustr'ate thef'mounting of "the".foraminous member' and vibrator; p

Figure 4is a detaile'd view of. another I classifier "accordring to the invention; partly "cut'awayno 2 illustrate detail; and

Figure Sis ageneral'side view of an apparatus according to theiinvention? illustrating" the" manner '"in which a number of classifiers accordingto the invention 'may be "mounted'in"series"for'purpo'ses of collecting a series of different "sized products" having predetermined size-weight characteristics.

Referring:now'more particularly to the drawings, the "general arrangement offthe components .ofrthe' material reduction mill in connection with which classifiers accord- :ingto "the present invention -may'be used to :advantage is illustrated in Figure 1.

.Thernill comprises "the"cylin'd'rical drum 10' mounted for rotation on"h'ollow:rtrunnions" 'housed in -the bearings arena 12. The'gear 13 "is mounted ronthedrum'ltl ":and'*is.drivent'by drive'pinion. (not'sh'own) to rotateithe drum: at the required speed. "Material'is fed'tothe' :drum, "for'instan'cei from .the" conveyor 14 'andfallsinto"the'feed "chute 15 and"i'nto the 'drum through thehollowttrunc'nion which" is' mountedin thebearing' 11.

Comminuted'rnaterial' is withdrawn'from the drum "10 in "an"airstrearnwhichJ is drawn through itheufeed chute 15-and across theanill to" entrain: and remove: particles below a' certa'in' .size weighf factor. The air streamiis passed through an=air circuit, which ontheidownstream side *bfthe 'rn'ill," cont'ainsa number. of"components"for separating products 'a'nd""'"-'delive'ring them for "dispos'al. ""lT'he" classifier of 'the' presentrinvention consists "of "two sections ofduct andis.indicated'fgenerally:on Figure l -by 'thenumeral 16. It? consists essentially of '"the" duct component "17, *the duct component: 18 and-the" coarse products collector 19. "Iheduct components 17'and' 18 may be constructed inlconventional manner andprefer- A'bovethe classifier '16 there may bemount'ed 'a ;by':paSS j gate25 which-consists simply "of a. gate-. establishingcom- -.munication betweeriltheiinterior ofitheaircircuit andthe ..:atmosphere by meanstofwhichzit is possible, by-adjust- .ninglthe' degreeofiopeningflhereof,toradjust the speed-.of the. airastream-passing: through" thei drum. andi-through-the classifier. :The iair stream.- is delivered towthe main prodmcts eollector=20 anditherrpassesithrough duct 21.toithe impeller? 22rdriven by:motor 23.-and is' exhausted. to .atmos o phere through the dust": collector 24.

t-The'fmil-lsithus described is'an' open circuit one, but it -=w'ill be appreciated-that the invention can be usedequally s Well in mills employing aclos'ed"aircircuit.

' Furning'now't'o' Figures 2; 3"and 4,' it will beobserved that the classifier 16 comprises duct components 17 and 18 which are mounted in particular relationship one to 1 another and are of particular shapes and sizes.

The upstream duct component 18 is of generally rounded or curvilinear cross section and is adapted, at its upstream end 26, to fit into the hollow trunnion on the outlet side of the mill forminga plane of connection 26a therewith. As showndnFigures 2 and 3, the duct comiJ POHBlJfi518'FiSIH3d-fi11in twot sections joined; at 27. This, however, is merely for convenience of manufacture and is uninnpontantninsofiar.;as;;the1present-invention is concerned. The duct component 18 is of generally expanding :cross sectional area and terminates azfiange 28 which is adapted to connect tothe flange v.29 on the duct component17 so' that a-planebfintersection 30 is formed between the two ductcamponents 17.. and 18.

The downstream duct component 17 is also of curvilinear or rounded cross section and may either be of uniformly cross sectional area, of expanding cross 'Section'aLarea orof diminish-ing'crosssectional area, 'depending upon what other components 'besides the classifier '16 'arepto be" in the .air circuit on' the downstream 'side 'of'the' drum-10.

The upstream duct component 'is-provided'on its lower :''side WithJ-the "opening 31 {which is covered by =the foraminous member32 =whi'ch, is illustrated as-being a .screen and which may, be a 'se-reen having. apre-detercrnined m'esh size'in the'event that oversize is to be revturn'ecl to the' -drum 10, 'or' may be formed from a'punched ."rnetalisheet-"in -the-='event that all-"material-withdrawn from the airstrearn' by i the classifier -is to" he collected. Secured in position below 'the foraminous member 32 is the" hopper 34 which-delivers coarse products collected thefein throtiglfthe rotary-air lock 35- whichis of convent-ional "design and-"isarranged "to deliver products 35' 'through the" deliverychute 36 'without permitting air to be' drawninto the hoppe134. This is accomplish'ed, for instance;bya'rotary'airlockhaving a number of rotating vanes 37. "Irr'the event th'ata screen of pie-determined rnesh size is-'=used'---'to *cover'the opening 31, normally 40==speaking there will" be a' vibrator 38 mounted more or less centrally upon the screen l32.

The dimensions of the duct component 18 and the -an gle' at which the" duct components 18 and 17 intersect each other is of critical importance to the invention. In the first place t-he duct component ls-must be arranged to lie withi. its lower-side at an angleofat least' 38 to the vertical 'in order -to= assure the?v free flow of material -withdrawn from) the airstream withinit, :back'. towards the drum 10. -Secondly; the area 'of the plane-w'of intersection 30 must be at least 1.5 times thearea of the plane of connection 26a. Thirdlyflthe'-length ofl'thelaxis of I the @duct: component -18, between'rthe; iplanerof connection ls2'6trsand thetplane: "ofrzintersectionw30, :musti-be *at least ...2.4:-.times 'thenmeamdiameter of {the plane; Ofr CGIlI'lGCfiOH r0651. Eounthly thetrduct components 17;;and--18-.must ..both 'bet,of.azgenerallyaroundedor curvilinear cross sec- ::tion. 1. Inthisrrespect-theytmaybe round, oval, etc.:.but ..=.should ..not....:be ..rectangulari for ,,maximum.,,.efliciency. 'Fift'hly, theanglecfiintersection. hetweenthecentral axes iofihe'rluct componentSlf17a'nd 18 must .be between.ab,out 394 and 128. f'SiXihlLlFthe centralu'axis ,i'of the? duct :cornponent 18;:when extended; mustrintersect' the outer wall of theduct component l7- to'iform an angle'fi with :f'thetangent "plane-at" the point of intersectionwhich is 6.3-nofilessthanabout 38. Finally,-"the duct-component "17 must be at least l'.5, timesaslongas the-rneandiameter of the "plane of: intersection '30.

The aboveilisted'critical features ina-thendesign of the ;.:classifier:=of1 thexinvention :are. I largely-empirical,-. although 70715111611? the; theory bf: then-separation. of: particles; in the 1:1 classifier: :according to size-Weight :factor; 118' considered, 121116 reasons for: the =.various: criticabfeatures. involved be- ,come apparent. .--.-In-v theory: the classifier, of; thezinvention is:believed to, operate as follows: 1 .The airstream passing. throu h. .the. drum ,10. will ,.en-

attain, all particleswithin. .the mill whose paths of travel intersect it, ,,and;the., si-ze-weight Ifactors whereof are .be- .".low..a given value. jglf...the.,.materialQbeing. comminuted is r lot-uniform specifics-gravity, density, andv physical form, the particles removed, fromthemlill-will .all ,bebelow a given size; however, many rocks, and. oresconsist ,of agglomerates "of two or more' difieren.t..materials..having different specific gravities, different physical forms and/or difierent...k1ensities in 'ltheir gprimarycrystalline "state, for instance, in thecaseof asbestos ore; the primary macewos crystals have a specific gravitywhich not -very much different .from the specific gravity .of the' host rock, 'but the primary crystals occur as long fibres having a relatively large surface area compared, for instance, to a sphere, and are thus much :more :easily picked :up by an'a'irstrearn than a theoretically round piece :of material of the same size would be. Thus, with most materials the particles picked :up by the airstream passing :across the drum of the mill will be either picked :up by the airstream or rejected by it, :d'epending mpon a factor'which is a function :Of the size-of the particle, the weight of the particle and the physical state thereof. For convenience of reference herein, I will refer to this factor as'being'the"size-weight factor of'the material. With respect to any given material this z-fa'ctor' is the one which determines whether a given particle will be picked -up and carried along by a stream of air of given velocity or whether "it will not bepicked up and carried along :by the-said stream'of air.

The particles picked 'up by the airstream crossing the drum are carried alongpast the'p'lane of connection 26a wherethey enter the duct 18. *In-order-to move along the duct 18, particles must change direction so-tha't the heavier of them will strike the lower surface 10f th'e ductand' lose some of theirmomentum. However, since the airstream within the lower portion of 'the duct 1'18 has a higher velocity than the airstream at the po'int o'f the plane of intersection due to the lower cross sectional area of "the duct, particles will be reentrained unless they are of such a large size-weight factor that-theinpresence within the lower portion of the duct 18 may be considered as having been brought about by causual circumstances. The entrained *particlesproceed up the duct 18 until they approach the plane of intersectionBl) at which point-the airstream undergoes a change in direction vandthe coarse particles (we are here concerned mainly with the coarse fraction of the products of the mill) will have vconsiderablemomentum because of the fact that the velocity of the airstream in duct 18 will normally be of the order of five to six thousand feet per minute. All the particles, therefore, exceptthe very .-.fine products, will tend to continue :straight ton and will strike the outer wall of the :duct component 17. The duct 1'7 is placed at such anyangle to the duct :18

that particles striking the outer wall thereof ,will lose 7 substantially all of their motion in the direction of the axis of the duct 17. Normally, however, they will rebound off the wall back into the airstream at which point they will either be re-entrained by the airstream flowing in the duct 17 or they will fall back and slide by gravity down the bottom side of the duct 18 from where they may be removed through the foraminous member 32. It might be mentioned at this point that, even if all of the material thus removed from the airstream is to be collected by the coarse products collector 19, it is essential to have either a screen or a punched plate over the opening 31 to preserve the flow characteristics of the duct 18; otherwise, the products collected will become seriously contaminated with fines.

Whether or not a particle placed substantially motionless in an airstream will be entrained or will settle out from the stream depends upon the aforementioned sizeweight factor, and by controlling the velocity of the airstream within the duct 17 the size-weight factor of the particles withdrawn from the stream and returned to duct 18 can be controlled within a surprising degree of accuracy. Furthermore, the coarse particles withdrawn an returned will b u t ntially free of fines and dust.

Turning to the critical features above mentioned and relating them to the above expressed theory of operation of the classifier, it will be apparent that since the airstream passing through drum 10 has picked up particles from a substantial stand still in the drum, the diameter of the duct 17 in the area of the plane of intersection 30 will have to be somewhat greater than the area of the stream as it leaves the drum; otherwise all the particles knocked out of the airstream by impinging upon the surface of the duct 17 will be re-entrained. It has been found in fact, that to get efficient separation, the area of the plane of intersection should be at least 1.5 times the area of the plane of connection 26a. Since the separating action at the intersection of the two duct components depends primarily upon each particle having gained enough momentum in the general direction of the axis of the duct 18 to impinge upon'the outer surface of desired to withdraw,

able. flow :conditions to return substantially to normal :the mean :diameter ofsithe .plane of connection 162:,

The duct component 17 must be long enough .to provide a substantial length of jductiin which the separating action can ta'ke place, and in general its lengthshould -not beless than 1.5 :ti'rnes the mean diameter-cof the plane of connection 30. The outer walltoftthetduct 17 'must form :asufficient: angle with the extendedaxis of the/duct 18 to :ensure that particles striking it 'will' lose substantially all their velocity in the direction of th'evaxis :of theduct 17. vIthas been foundfin practicetthatr an angle greater than about 38 :willcnsure an efficient separating actioninthis respect. Similarly'the -axes of- .the ducts 17 and 18 should meet at an :angle which 'is between about 94 and 128 Ito rensure a sufficient-change in direction ofithe-airstream.

The duct sections 17 and 18 should be curvilinear :or rounded in cross section because, .if tthey'were rectangular 'lwlth sharp corners, various secondary ."fiow conditions would arise, interfering with the accurate separation action of :thetclassifienand .tendingftozdeposit fines and so forth, as well as the products-which-it is If, however, a substantial quanttityroflfinescambe tolerated :in the coarse-product to'be collected, ,the duct can be made With .a square .or rectilinear :cross FSBClZiOlI.

IIihe classifier according :to "the invention may, if :desired, be arranged in series with =21 number 'ofzother such :classifiers, as illustrated {by .Eigure 5 whichwshows three classifiers according to :the invention, arranged in series 10 collect three products of gradually decreasing .size in. the hoppers 40,' 41"and42. In :this case it will be observed that component "43 :is the first component, and component 44. is-the second-component with :respect to .component 43 but ,is the first eornponent with respect :to component 45, while component 45 ;is ;likewise the second component withrespect totcomponent 44 but the first.componentiwith respect:to component 46. The type ,of arrangement. illustrated in Figure 5 may be :used to advantage .in cases where differential grinding :may rbe usedloibene'ficiate .the materialtundergoing comminution,

to simplify subsequent purification steps, or, for instance, in the making of brasive productswhere it is desired to get products in different size ranges from a material undergoing reduction which is of a uniform composition and densitv. In the latter case the arrangement illustrated in Figure 5 may be used to produce a number of products, each one of which will have an accurately Eegulated particle size and will be substantially free from ust.

From the foregoing it will be appreciated that the present invention produces a simple effective classifier for carrying out the objects aforesaid. The foregoing description, however, is not to be construed in any limited sense, the invention and its scope being defined in the appended claims.

What I claim as my invention is:

1. A classifier for removing and recovering from an airstream entrained solid'particles having a size-weight factor above a predetermined value, said classifier comprising (a) two components of duct arranged with their longitudinal axes in substantially the same plane and intersecting at an angle, said components meeting along a plane of intersection; said classifier being adapted to form part of an air circuit, and receive an air stream at a plane of connection thereof with a component of said air circuit upstream of said classifier; the area of said plane of intersection of said ducts being at least about 1.5 times the area of said plane of connection; the angle between the longitudinal axes of said two components being be tween 128 and 94; the longitudinal axis of said first component, when extended, intersecting the outer wall of said second component to form an acute angle with the tangent plane at the point of intersection of not less than about 38; the length of the longitudinal axis of said first component between said plane of connection and said plane of intersection being at least 2.4 times the mean diameter of said plane of connection; said second component being at least 1.5 times as long as the mean diameter of said plane of intersection; said first component having a lower side downwardly inclined towards the upstream end thereof to provide for downward flow thereover of deposited material, said lower side having an opening formed therein; (b) a foraminous member generally corresponding in size and shape with said opening and secured in position to cover said opening, and (c) means arranged beneath said foraminous member for collecting particles which pass through saidforaminous member.

2. A classifier as defined in claim 1 in which the angle between the axes of said two components of duct is substantially 125 and the axis of said second component of duct is substantially vertical.

3. A classifier as defined in claim 1, in which said two components of duct are generally curvilinear in cross section throughout their length.

4. A classifier as defined in claim 3, in which the angle between the axes of said two components of duct is substantially 125 and the axis of said second component of duct is substantially vertical.

5. A classifier as defined in claim 4, in which said second component of duct is of gradually diminishing cross section.

6. A classifier as defined in claim 1, in which said foraminous member is 'a punched sheet having foraminations therein large enough to allow the largest of the particles removed from the airstream in the classifier to pass therethrough.

7. A classifier as defined in claim 1, in which said foraminous member is a screen of predetermined mesh size, and comprising vibrator means mounted on said means for preventing passage of air upwardly into said hopper comprises a rotary air lock.

10. A classifier as defined in claim 1, in which the area of said plane of intersection is between about 1.5 and about 3 times the area of said plane of connection.

11. A classifier comprising a plurality of components of duct, including a first component and a last component, said components being arranged in series with their longitudinal axes intersecting at angles, each component of duct meeting the subsequent one at a plane of intersection; said first component being adapted for connection to a component of an air circuit, forming a plane of connection with said component; the area of the plane of intersection of said first component with the subsequent one being between about 1.5 and 3 times the area of said plane of connection, each succeeding plane of intersection being between about 1.5 and 3 times the preceding one; the axes of connected components of duct intersecting at an angle of between 94 and 128; the axis of said first component of duct being at least about 2.4 times the mean diameter of said plane of connection; the axis of each succeeding component of duct except said last component being at least 2.4 times the mean diameter of its plane of connection with the preceding component of duct; the axis of said last component of duct being at least about 1.5 times the mean diameter of the plane of connection thereof with the preceding component of duct; said first component of duct leading upwardly at about 38 to the horizontal, each succeeding components of duct extending generally upwardly; openings on the lower sides of each of said components of duct except said last component; foraminous members covering said openings; and means disposed beneath said foraminous members for collecting material passing therethrough.

12. A classifier as defined in claim 11, in which said components are all of generally curvilinear cross section. 13. A classifier as defined in claim 11, in which said foraminous members are screens of predetermined mesh size, each succeedmg screen being of smaller mesh size than the-immediately preceding one.

14. A classifier as defined in claim 13, including vibrator means mounted on each of said screens.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 550,911 Fell Dec. 3, 1895 743,814 Boyd Nov. 10, 1903 1,708,195 Stebbins Apr. 9, 1929 2,162,392 Solomon et al. June 13, 1939 

